1. Field of the Invention
The present invention relates to a sound field offset device which is applied, in sound reproducing, to a sound field where reflected sound waves and the like may adversely affect frequency characteristics and locality of acoustic images sensed at a listening position.
2. Description of the Prior Art
In sound reproducing, reflected sounds may occasionally be a major cause disturbing the frequency characteristic at a listening position, and impeding a sense of locality of acoustic images. In a car's closed space, in particular, direct sound waves are greatly disturbed by first or second reflected sounds existing in a sound field inside a car, because the size of the car's space is small, and reflector walls such as glass windows usually exist nearby the listening position.
FIG. 1 shows a calculated value of an echo pattern changing with time in the sound field of the car's internal space. It can be seen that a major group of reflected sound waves concentrates with a delay of 2 ms to 3 ms in succession to the direct sound wave. The order of the delay time noticed in the above response is similar to the one derived from the spatial separation between both ears. These reflected sound waves interfere with the direct sound wave in phase, disturb the frequency characteristics at the listening point, and destroy the sense of locality of acoustic images. A graphic equalizer employing analog filters which has been conventionally used as a sound field offset device cannot improve the sense of locality of acoustic images. The reason for this is that although the graphic equalizer can offset the amplitude characteristic of sounds up to a flat or any required characteristic, it cannot control the phase characteristic of sounds. Recently, an attempt to offset sound field has been made by controlling the phase characteristic of sounds by means of a digital filter technique. Such a technique has achieved an improvement in the frequency characteristic of a sound field where the effect of reflected sound waves is noticeably strong and an improvement in the sense of locality of acoustic images in an asymmetrical sound field such as the sound field in the car's space.
Since high frequency response plays an important role in the locality of acoustic images, this response should also be subjected to the sound field offset even when the sound field offset is performed by means of the digital filter technique. Signal processing up to the audio frequency band, however, requires a higher sampling frequency and fast arithmetic speed in the filter, thus increasing a burden on hardware design. Although it may be theoretically possible to handle the entire audio frequency band with the digital filter, a great deal of difficulty may arise in implementing such a scheme from the standpoint of cost and feasibility.